Axial deposition and outside deposition are widely known methods for forming an optical fiber preform, and involve obtaining a porous glass base material by depositing glass fine particles generated by hydrolyzing a silicon compound such as silicon tetrachloride in an oxyhydrogen flame.
FIG. 1 schematically shows an apparatus for manufacturing the porous glass base material using the outside deposition technique. In the outside deposition technique, dummy rods 2 fused to both ends of a core material, referred to as the target, are held by chucks 3 and rotated, and a burner 4 for silicon deposition is moved back and forth across the longitude of the target 1, thereby depositing the glass fine particles generated in the flame 5 onto the target 1 as a layer.
The porous glass base material 6 formed in this way is tapered at both ends, and the dummy rods 2 are attached to the tips at both ends.
Next, as shown in FIG. 2, the porous glass base material 6 undergoes a sintering process that involves sintering in a heating furnace 7 to change the porous glass base material 6 into transparent glass, thereby obtaining the base material ingot. Usually, the sintering involves connecting the dummy rods 2 of the porous glass base material 6 to a suspended sintering rod 8, which is suspended vertically in the core tube 9, and moving the porous glass base material 6 vertically relative to the heating furnace 7. As a result, the change to transparent glass gradually occurs from one end toward the other end, thereby forming the base material ingot.
In the process for forming the transparent glass, it is common for the tapered portion where the sintering begins to be completely finished while the tapered portion at the opposite end retains a portion that is not completely changed to glass, i.e. an unsintered portion. The reason for this is that the sintering of the base material ingot is performed with the porous glass base material 6 being vertically suspended, and therefore, when the portion where the sintering ends is reached, the entire weight of the ingot formed by the glass resulting from the hydrolysis is added. The tapered end is left unsintered in order to prevent this weight from extending the thin-diameter portion. Therefore, as shown in FIG. 3, the sintered base material ingot 10 includes the unsintered portion 11.
Japanese Patent Application Publication No. 2003-089541, for example, discloses a method that involves obtaining an optical fiber by drawing, as-is, the base material ingot having the unsintered portion in the upper region manufactured as described above. This base material ingot after sintering, as described in the above Publication, has an outer diameter that differs along the longitude thereof due to the gravitational force exerted on the base material ingot during sintering. Furthermore, the base material ingot usually has a diameter of 100 mm or more, and base material ingots manufactured recently often have diameters exceeding 170 mm, or 100 kg.
In order to draw as-is a base material ingot with such a large diameter and differing outer diameter across its longitude, it is necessary to increase the size of the drawing furnace to house the differing outer diameter. Furthermore, the gas seal mechanism of the portion that inserts the base material ingot to the drawing furnace becomes complicated.
In many drawing furnaces, a preform is used whose diameter is 80 mm and whose external diameter is within a range of ±1% of the average outer diameter. In order to obtain this preform, the base material ingot that has been changed into transparent glass is roughly extended to have a thickness near the diameter of the preform to be used for drawing in the extension process, i.e. the extension in the furnace, using an electric furnace as shown in FIG. 4.
In FIG. 4, the base material ingot 10 is lowered by a furnace extension lowering rod 12 into an electric furnace 14 including heaters 13. Rough extension is performed by adding a tensile force to a furnace extension pulling rod 16 whose bottom end is connected to pulling rollers 15.
The roughly extended preform is extended until reaching a state where the diameter across the entire length thereof differs by no more than 1% from the external diameter, using a glass lathe, and the optical fiber is formed by performing the drawing process after polishing the surface of the preform with a flame.
A flame is used for the extension by the glass lathe and there is a limit on the heating ability thereof, and therefore it is difficult to correct large differences in the outer diameter. Therefore, with the rough extension during the furnace extension process, the difference in diameter of the preform if preferably made as small as possible.
In a widely known technique, the base material ingot is suspended vertically during the furnace extension process as well, and the preform extending below the heating furnace is drawn. Therefore, the dummy rod from which the base material ingot is suspended is used for suspension during both the sintering process and the furnace extension process. In this case, the portion of the base material ingot that has been completely changed to glass is the portion that begins the furnace extension first, and the portion that has yet to be changed to glass, which is arranged toward the top, gradually proceeds to undergo the furnace extension.
When furnace extension is performed with this arrangement, as shown in FIG. 5, the diameter of the preform is particularly large in the region where the extension ends. Since the portion changed to glass and the unsintered portion have different thermal conductivity and specific heat, the extended portion changes more, even at the same temperature. Furthermore, if the feeding amount of the base material is increased too much, the base material can tear at the border where the change to glass occurs.
One technique for preventing the upper portion from being extended first in the region where extension ends, as described in Japanese Patent Application Publication No. H07-033463, involves placing a thermal insulating jig on the upper portion of the base material ingot. However, effort is required to place the jig prior to the extension, and the jig makes it difficult to sufficiently achieve the effect of restricting the diameter change, particularly for base material ingots with large diameters greater than or equal to 150 mm.